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1. Field of the Invention
This invention relates generally to data communications and, more specifically, to techniques for implementing redundant data compression in communication or storage systems which are vulnerable to errors. Such systems may be used to compress signals such as speech, audio, and/or video.
2. Description of Prior Art
Many communication systems, such as cellular telephones, personal communications systems, voice-over-Internet protocol (VoIP), as well as audio or video over packet-based networks, rely on electromagnetic or wired communications links to convey information from one place to another. These communications links generally operate in less than ideal environments with the result that fading, attenuation, multipath distortion, interference, network congestion, network delay, and other adverse propagational effects may occur. Similarly, such less than ideal environments may also occur in storage systems. In cases where information is represented digitally as a series of bits, such propagational effect may cause the loss or corruption of one or more bits. Oftentimes, the bits are organized into frames, such that predetermined fixed number of bits comprises a frame. A frame erasure refers to the loss or substantial corruption of a bit or a set of bits communicated to the receiver.
To provide for an efficient utilization of a given bandwidth (or storage space), communication (or storage) systems directed to signal communications (or storage) often use signal coding techniques. Such signal can be for example speech, audio, or video. Many existing signal coding techniques are executed on a frame-by frame basis. For speech coding, such frame is about 10-80 milliseconds in length. The signal coder extracts parameters that are representative of the signal. These parameters are then quantized and transmitted (or stored) via the communications channel (or storage medium).
If a frame of bits is lost, then the receiver has no bits to interpret during a given time interval. Under such circumstances, the receiver may produce a meaningless or distorted result. Although it is possible to replace the lost frame with a new frame, estimated from the previous or the next frame, this introduces inaccuracies which may not be tolerated or desirable in the context of many real-world applications. In predictive coding systems, such errors will then propagate to all the future frames. The result is a poorly-reconstructed and distorted signal.
The problem of packet loss in packet-switch networks employing speech, audio, or video coding techniques, is very similar to the problem of frame erasure in the context of Internet, or wireless communication links. Due to packet loss, a signal decoder may either fail to receive a frame or receive a frame having a significant number of missing bits. In either case, the signal decoder is presented with essentially the same problem: the need to reconstruct a signal, despite the loss of compressed signal information. Both frame erasure and packet loss concern a communication channel (or storage medium) problem which results in the loss of transmitted (or stored) bits. Therefore, for purposes of the present disclosure, the term xe2x80x9cframe erasurexe2x80x9d may be deemed synonymous with xe2x80x9cpacket lossxe2x80x9d.
An improved signal coding technique encodes signal parameters into a plurality of overlapped super-frames, each super-frame having a predetermined number of bits. These encoded signal parameters may be decoded into plurality of non-overlapping frames. Redundant information derived from an overlapping portion of the super-frames is used to reconstruct frames which have been subjected to frame erasure. This technique reduces or eliminates the need for prior art error concealment schemes, which are commonly based on interpolation or extrapolation (or both), to reconstruct an approximation of the signal within the erased frame or frames. The invention can also be utilized as an enhancement layer added to standard or other prior art coding systems. In such cases, an existing, possibly standard, coding scheme may operate as a base layer, and the present invention can be used as enhancement layer for less than ideal communication (or storage) environments.